Optical fiber connectors are an essential part of substantially any optical fiber communication system. For instance, such connectors may be used to join segments of fiber into longer lengths; to connect fiber to active devices such as transceivers, detectors and repeaters; or to connect fiber to passive devices such as switches and attenuators. The central function of an optical fiber connector is the maintenance of two optical fiber ends such that the core of one of the fibers is axially aligned with the core of the other fiber; and consequently, all of the light from one fiber is coupled to the other fiber as efficiently as possible. This is a particularly challenging task because the light-carrying region (core) of an optical fiber is quite small. In single mode optical fibers the core diameter is about 9 microns where 1 micron=1×10−3 mm. For multi-mode fiber the core can be as large as 62.5 to 100 microns, and hence alignment is less critical. However, precision alignment is still a necessary feature to effectively interconnect the optical fibers.
Another function of the optical fiber connector is to provide mechanical stability and protection to the junction in its working environment. Achieving low insertion loss in coupling two fibers is generally a function of the alignment of the fiber ends, the width of the gap between the ends, and the optical surface condition of either or both ends. Stability and junction protection is generally a function of connector design (e.g., minimization of the different thermal expansion and mechanical movement effects). An optical fiber connector typically includes a small cylinder of metal or ceramic with a glass or plastic fiber installed along its central axis. This cylinder is typically referred to as a ferrule. The support structure around the ferrule and the mechanism (typically a spring) pushing the ferrule into an opposing ferrule comprises the operating sections of an optical connector.
In a connection between a pair of optical fibers, a pair of ferrules is butted together—end to end—and light travels from one to the other along their common central axis. In this conventional optical connection, it is highly desirable for the cores of the glass fibers to be precisely aligned in order to minimize the loss of light (insertion loss) caused by the connection; but as one might expect, it is presently impossible to make perfect connections. Manufacturing tolerances may approach “zero,” but practical considerations' such as cost, and the fact that slight misalignment is tolerable, suggest that perfection in such matters may be unnecessary. What is critical however is stability across the anticipated operating environment the fiber joint will be installed into.
Historically, due to manufacturing costs and design features, optical connectors have tended to be manufactured as an assembly of loose components, many of which are manufactured from plastic. For high performance connectors intended for single mode I application, the need to tune out the eccentricity of assemblies has been required and until the introduction of this invention, there has not been a method to utilize all metallic or ceramic structures to achieve tuning and performance in extremely harsh or severe, environments that exceed the operational characteristics of plastics. Tuning has been enabled in the past when the ferrule support structure engages the connector housing. This is an undesirable effect as the housing becomes an integral element in tuning and if the housing is removed or replaced, tuning is in effect lost.
One popular design of an optical fiber connector is shown in U.S. Pat. No. 4,793,683; and its basic components comprise a precision molded plastic conical plug having an optical fiber centered therein, a compression spring disposed about a cylindrical portion of the plug, and a retention collar surrounding the plug and spring. The collar includes external threads that enable it to couple with another connector via a fixture having a precision molded alignment sleeve whose shape is best described as “biconic.” This design has been superseded by the connector shown in U.S. Pat. No. 4,934,785 which comprises a cylindrical plug, a base member that holds the plug, a compression spring, and a cap that surrounds the plug and spring. In this design, only the cylindrical plug needs to be of high precision and is typically made from a ceramic material. When joining two of these plugs together, an alignment sleeve is used, which comprises a split, thin-walled cylinder made of metal, ceramic or even plastic material. This alignment sleeve need not be as precise as the above-described biconic alignment sleeve.
And while the above connectors perform satisfactorily, further improvements were desirable. For example, because of the growing acceptance of optical fiber as the transmission media of choice for analog and digital data, the need to provide higher density interconnection arrangements emerged. All of the above-mentioned simplex optical connectors are constructed in such a way that the ability to stack a large number of them together is limited by the need to manually grasp both sides during insertion and removal from a receptacle or coupling device. Known duplex optical connectors, such as the one shown in U.S. Pat. No. 4,787,706, also require manual access to the opposite sides of its housing during removal from the receptacle or coupling device which precludes high density optical fiber interconnection arrays.
With these latter desires in mind, reference is made to the art of electrical connectors where, perhaps, the most used and accepted connectors are the ones known as RJ11-type plugs/jacks that are typically used in corded telephone products. These connectors have achieved widespread acceptance because they are inexpensive, they operate reliably, and customers readily understand their operation. However, because of the high precision and low insertion loss requirements associated with optical interconnections (particularly between small numerical aperture single mode fibers), RJ11-type designs have been ‘unacceptable for optical connectors. Examples of such electrical connectors are disclosed in U.S. Pat. Nos. 3,761,869 and 3,954,320.
Recognizing the engineering challenge posed by the alignment of two very small optical fiber cores, it is still desirable to provide connectors, which are smaller, less expensive, and yet more convenient for customers to manipulate. Such connectors would be of great commercial importance. Such a design is disclosed in U.S. Pat. Nos. 5,481,634, 6,293,710 and 6,287,018 where-in a plastic material is used to manufacture a housing assembly. Said housing assembly closely resembled the RF11 type noted above. The latching system is integrated as a single cantilever assembly. Said connector identifies the use of the fiber-holding structure which comprises a cylindrical plug and a base member which holds an end portion of the plug. The base member is generally cylindrical, but it includes a flange around its circumference at one end thereof. A spiral compression spring surrounds the base member with one end of the spring pressing against the flange and the other end pressing against an interior surface of the housing. Preferably, the ferrule has a circular cross section and whose axial passageway (capillary) is substantially concentric with the outer cylinder surface. Additionally, the flange is adapted to enable the base member to fit into the housing in a number of different stable positions so that rotating the base member to orient the fiber eccentricity in a predetermined direction can minimize fiber eccentricity optical performance degradation. In the preferred embodiment of the invention, a square flange is used although in later patents the design feature has incorporated hexagonal eccentricity adjustment features. The description of the product in these patents by AT&T Corp. Anderson, et al is for what is commonly known as the LC connector.
The LC connector delivers all of the features desired by a majority of the optical connector applications with the exception of performance in extremely harsh environments, and an ability to directly inspect internal elements of the connector or replace external components of the connector without losing the tuning or eccentricity compensation. These are limited by the very nature of the connector itself, either a one piece outer housing or a complex assembly of plastic components that by nature are damaged when repairs are attempted. Such plastic components are also by nature unstable at elevated temperature where plastic elements begin to out gas and become brittle. Within the U.S. Pat. No. 5,481,634 mention is made of connector construction using metallic body materials. However, the design suggested requires a multi-sectioned housing rather than a one piece housing. With the commercial market's desire for a severe environment version of the LC connector that retains its tuning or eccentricity compensation, if such a connector with a single piece metallic or ceramic housing could be designed to be manufacturable at a reasonable cost, it would hold extremely high value in various applications, such as aerospace, chemical or pharmacology applications.